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Re: [loopantennas] Re: loop antenna - has to be in resonance?

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  • Andy
    ... Well, no, that is not true. If there is some amount of transmission line (coax, feedline, etc.), and if it is a low impedance line driving a high impedance
    Message 1 of 4 , Nov 30, 2012
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      > ... This is true for all RF circuits - as long as you have a low
      > impedance driving a high impedance, you will have little or no reflection,
      > low SWR, and matching is not required.

      Well, no, that is not true.

      If there is some amount of transmission line (coax, feedline, etc.),
      and if it is a low impedance line driving a high impedance load, then
      you WILL get reflection, and high SWR.

      But if the line lengths are short and the frequencies low enough so
      that the length is electrically short, then you can treat it as a low
      frequency circuit, and then the SWR that is on that short line may not
      bother you. Then, you can get maximum voltage at the load by
      connecting a high impedance load to the low impedance source. But you
      still do not get maximum power transfer except when the impedances
      match.

      For reception of radio signals sent short distances, impedance
      mismatches (and consequent reception inefficiency) is OK because your
      received signals are so strong.

      When the received signals are weaker, like they usually are in radio
      communications, it's a different story. We can often tolerate the
      mismatch+inefficiency on MF and HF because the atmospheric noise tends
      to dominate the signal-to-noise and everything is greater than the
      receiver's own noise, so some loss in received power (due to the
      mismatch) doesn't hurt the S/N ratio. But once you get up into VHF
      and above, you really need to match impedances between antenna and
      receiver because the receiver's noise is the limiting factor. A
      mismatch between the antenna and receiver causes signal loss and
      degraded S/N, and likely inability to receive weaker signals that you
      should have been able to receive.

      > Add a fixed resistor in series with the parallel RC

      I am guessing you meant to write inductor or capacitor, not resistor.

      Andy
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